1. Field of the Invention
The present invention relates to an electrical discharge machine concerned with workpiece finishing and, in particular, mirror finishing.
2. Description of the Background Art
FIG. 13 shows a conventional electrical discharge machine. In the drawing, a machining gap 1 is formed between an electrode and a workpiece. A direct-current power supply 2 has a value of about 80 to 100 V. Also seen in the Figure is a current limiting resistor 3, a switching device 4 which applies a voltage and cuts off an output current, a drive circuit 5 which drives the switching device 4, and a capacitance 6 formed in the portion of the machining gap 1 where the electrode and workpiece are opposed to each other.
During machining, a voltage is applied to the machining gap 1 formed by the electrode and workpiece to machine the workpiece. Namely, the switching device 4 is first turned on by the drive circuit 5 to apply the voltage to the machining gap 1. This applied voltage causes a discharge to take place in the machining gap 1. The discharge is detected by a discharge detection circuit (not shown) and, after a given current pulse duration (pulse width) has elapsed, the switching device 4 is turned off to supply a current pulse of given pulse width. Subsequently, after a given off time, the switching device 4 is turned on again to apply a voltage. The above operation is repeated to machine the workpiece.
It is to be understood that in such machining process, the machining capability and machined surface roughness will depend on the current value of the current pulse supplied to the machining gap 1. In other words, when the current value of the current pulse rises, machining speed increases but machined surface roughness is deteriorated. Also, when the current value of the current pulse falls, machined surface roughness improves but machining speed decreases. Namely, the change in current value of the current pulse provides a desired machining characteristic.
The pulse current value supplied to the machining gap 1, which is determined by the voltage of the direct-current power supply 2 and the value of the current limiting resistor 3, is generally controlled by switching the current limiting resistor 3.
In finishing, which involves a machined surface of good quality, a large value is selected for the current limiting resistor 3. In this case, the change in opposed area of the electrode and workpiece in the machining gap 1 results in a great change in current pulse waveform. Namely, when the opposed area of the machining gap 1 increases, a capacitance is formed in the machining gap. Such an increase in capacitance not only increases a capacitor discharge component 31 occurring prior to a direct-current arc component 30, as shown in FIG. 14(a), but also results in the occurrence of a current waveform in which the arc is cut off after the capacitor discharge 31 and the direct-current arc component 30 does not exist, as shown in FIG. 14(b). When such direct-current arc cut-off, generally referred to as a pulse crack phenomenon, takes place, the workpiece is machined by a capacitor discharge component 31 that is extremely short in pulse width, whereby machining speed reduces and the electrode is consumed significantly. Also, there is a tendency that electrode consumption causes the electrode surface to be toughened and machined surface roughness to be reduced.
The direct-current arc cut-off, i.e., pulse crack phenomenon, is likely to take place when the capacitor (capacitance) is larger and the current limit resistance value is greater. Specifically, when the direct-current power supply 2 is 80 V, the limiting resistor 3 is 10.OMEGA. or larger (current value 8 A or less) and the capacitance formed in the machining gap is 1000 pF or more, pulse crack is likely to occur and the machining characteristic tends to deteriorate significantly. Particularly when the current value is 5 A or less, this tendency is important.
In the meantime, as a method for overcoming electrode consumption and improving machining speed, Japanese Laid-Open Patent Publication No. SHO 50-78993 discloses that a series circuit of a not more than 20 .mu.H inductance and a not more than 2 .mu.F capacitor is connected in parallel with a machining gap. This publication teaches that, since machining speed reduces especially at the inductance of 20 .mu.H or higher under the condition that the machining current is 20 A, the most desirable inductance value is 15 .mu.H or lower.
Under the machining condition that the current value is high (not less than 20 A), as indicated in the publication, the pulse crack phenomenon is unlikely to occur naturally. Therefore, an inductance of 20 .mu.H or higher will result in the deterioration of the machining characteristic as described above. However, research by the inventor has shown that when the current value is low, i.e., not more than 8 A (finishing current value), and the inductance is 20 .mu.H or less, the pulse crack phenomenon takes place and leads to an increase in electrode consumption and a reduction in machining speed. As a result, the increase in inductance improves the machining characteristic.
If the machining current is low and the opposed area of the electrode and workpiece increases in the conventional electrical discharge machine arranged as described above, pulse crack where the direct-current arc is cut off occurs frequently. As a result, there is a decrease in machining speed, an increase in electrode consumption, and a reduction in machined surface roughness.
Also, if a series circuit, comprising an inductance of not more than 20 .mu.H inductance and a capacitor, is connected in parallel with the machining gap as disclosed in Japanese Laid-Open Patent Publication No. SHO50-78993, the pulse crack phenomenon often happens especially in machining where an electrode area is large under a finishing condition that the pulse current value is 8 A or less. As a result, there will be an increase in electrode consumption and a reduction in machining speed.
It is to be noted that as another conventional example, Japanese Laid-Open Patent Publication No. SHO50-103791 discloses an arrangement wherein machined surface roughness is improved by connecting a circuit, having a reactance, in series or parallel with a machining gap and by performing machining so as to be tuned and resonated to fundamentals or harmonics in alternating-current components included in the discharge current of a desirable discharge. However, since this example used a capacitor of about 100 pF, the current pulse width is decreased to an extremely small value, thereby increasing electrode consumption significantly.
It is accordingly an object of the present invention to solve the above described problems of the conventional art by providing an electrical discharge machine which can suppress pulse crack in finishing where an electrode area is large, in order to reduce electrode consumption and improve machining speed and machined surface quality.
It is another object of the present invention to provide an electrical discharge machine which can supply a current pulse having a low peak and short pulse width stably, especially in mirror finishing, in order to improve the surface roughness in a large area.
It is a further object of the present invention to provide an electrical discharge machine which consistently ensures optimum finishing in response to the changes in electrode area and machining conditions.